The present invention generally relates to electric machines which may be employed as starter-generators.
In many types of vehicles, an electric machine may be employed, in a motor mode, to start an engine. After the engine is running, the electric machine may be employed, in a generator mode, to extract power from the engine and to supply electrical power to various electrical loads of the vehicle. It has been found advantageous to employ brushless wound-field electric machines as starter-generators, particularly in vehicles such as aircraft or ground vehicles.
A typical wound field starter-generator may be operated in a start mode with AC power supplied to an exciter stator through an excitation control unit. An exciter rotor may produce AC excitation current which may be transferred, through rotor-mounted diodes, as DC current in a main rotor of the electric machine. Additionally, controlled frequency power may be supplied to a main stator. The controlled frequency power may be provided through a high-power inverter unit which may vary the frequency of the input power as a function of rotational speed of a rotor of the electric machine. The frequency of the power to the main stator varies so that the electric machine produces torque throughout its speed range (i.e., from start-up to starter drop-off speed).
There are alternative types of starter-generators that do not contain rotor-mounted diodes and have a direct connection between the exciter rotor windings and the main rotor windings. In either of these types of starter-generators, excitation power may be provided through position-controlled (i.e., speed sensitive) controllers so that torque may be produced throughout a speed range of the electric machine.
In either of these types of starting systems, there is a need for high-power inverters which may deliver controlled frequency power to the electric machine. Such inverters may add cost and weight to a vehicle.
As can be seen, there is a need for a system of operating a wound-field electric machine, in a start mode, without a requirement to supply controlled frequency power to the electric machine. More particularly, there is a need for such a starting system which may be operated with introduction of constant frequency power to a main stator of the electric machine.